Nitrate is found only in small amounts in fresh domestic wastewater but in the effluent of nitrifying biological treatment plants nitrate may be found in concentration up to 30mg nitrate as nitrogen/L. It is essential nutrient for many photosynthetic autotrophs and in some cases has been identified as the growth-limiting nutrient. Generally 10mg nitrate nitrogen/L of drinking water is recommended to prevent disorder called methemoglobinemia in infants. Determination of nitrate nitrogen is difficult because of the relatively complex procedure. Although UV technique that measures the absorbance of nitrate at 220nm is suitable for measuring nitrate in wastewater.
This technique is useful for the screening of sample that has low organic content that is uncontaminated natural water and potable water supplies. The nitrate calibration curve follows Beer's Law up to 11 mgN/L. Measurement of UV absorbance at 220nm enables rapid determination of nitrate. Because dissolved organic matter may also absorb at 220nm and nitrate does not absorb at 275nm, a second measurement made at 275nm may be used to correct nitrate value.
· Stock nitrate solution: Dissolve 0.7218gm KN03 in water and dilute to 1000m1 (1ml = 100mcg nitrate nitrogen). Preserve this with 2ml chloroform/L.
· Standard nitrate solution: Dilute 100 ml stock solution to 1000 ml distilled water (1 ml= 10mcg nitrate nitrogen').
· Take different aliquots of standard solution from 0.4, 0.8,………, 2.0 ml and one blank.
· Take three different aliquots (0.4, 1.2, 2.0ml) of unknown sample.
· Make up the final volume to 5ml with distilled water.
· Take the O.D. of all the tubes at 220nm (if organic matter is present take the O.D. at 275 nm also)
· Prepare a standard graph and plot the absorbance of unknown to find the concentration